Polymeric film

ABSTRACT

The invention relates to a polymeric film composed of an optically anisotropic material made from a cross-linked synthetic resin composition comprising a polymer network, being obtainable by copolymerization of a mixture comprising 
     (a) at least one monomer or oligomer, each of said monomers or oligomers having at least two polymerizable functional groups selected from the group consisting of (meth-)acrylate ester, epoxy and vinyl ether, 
     (b) at least one achiral liquid crystalline monomer or oligomer, each of said monomersloligomers having mesogenic groups and one polymerizable functional group selected from the group consisting of (meth-)acrylate ester, epoxy and vinyl ether.

This application is a divisional application of Ser. No. 08/894,181,filed Jan. 2, 1998, now U.S. Pat. No. 6,096,241.

SUMMARY OF THE INVENTION

The invention relates to a polymeric film composed of an opticallyanisotropic material made from a cross-linked synthetic resincomposition comprising a polymer network, being obtainable bycopolymerization of a mixture comprising

(a) at least one monomer or oligomer, each of said monomers or oligomershaving at least two polymerizable functional groups selected from thegroup consisting of (meth-)acrylate ester, epoxy and vinyl ether,

(b) at least one achiral liquid crystalline monomer or oligomer, each ofsaid monomers/oligomers having mesogenic groups and one polymerizablefunctional group selected from the group consisting of (meth-)acrylateester, epoxy and vinyl ether,

(c) a photoinitiator,

(d) additives selected from inhibitors and accelerators,

(e) optionally at least one chiral component, and

(f) optionally at least one liquid-crystalline mono- or dithiolcompound, and to a display device comprising such a polymeric film.

A similar display device is described in European Patent 0 246 842.

Chirality in liquid-crystalline materials leads to rotation of themolecules in a direction perpendicularly to their longitudinal axis. Inthe case of liquid-crystalline materials in the so-called cholestericphase, the pitch of the rotation is 0.1 to 1 μm. For application in, forexample, datagraphic displays using multiplex drive, a larger pitch ofthe order of magnitude of the cell thickness or even more of the displaydevice is desirable. Such a pitch, also called cholesteric pitchreferring to chiral nematic or cholesteric LCs, is obtained by adding achiral compound which also can be a liquid-crystalline compound itselfas a dopant to a nematic liquid crystal. With such materials,supertwisted nematic (STN) liquid-crystal display devices aremanufactured, the total twist of the molecular axis across the cellbeing, for example, between 180° and 270°. Such display devices have thedisadvantage that the optical properties depend to a large extent on thewavelength, of the light so that a high contrast and a colorless image(black/white instead of e.g., blue/yellow) is difficult to attain. Saiddisadvantage can be overcome in a known manner by using a combination oftwo identical cells, one of which contains left-handedliquid-crystalline material and the other contains right-handedliquid-crystalline material. When the molecular axis at the front of thesecond cell extends perpendicularly to the molecular axis at the rear ofthe first cell the wavelength dependence of the optical properties iscompletely compensated. However, as a result of this second cell theliquid-crystal display device becomes heavier and less compact.According to a simpler alternative, the second cell is replaced by auniaxial foil having an adapted birefringence. In this case, thecompensation of the wavelength dependence is not complete, resulting inthe display device exhibiting a contrast reduction and a certain degreeof color e.g. in the voltageless state. Another alternative consists inthe use of a twisted stack of uniaxial foils. This solution gets closerto the ideal situation (a twist and a birefringence which are equal tothe twist and birefringence of a supertwisted nematic liquid-crystaldisplay device) as the number of foils increases. However, this leads toa considerably more complicated production process. Instead of a foil,it is alternatively possible to use a birefringent layer on a suitablesubstrate. In European Patent Application 91-0 007 574 a description isgiven of liquid-crystalline polymer materials having a chiral dopant inthe form of a copolymerizable monomer. Such polymer materials are linearand have side groups which exhibit liquid-crystalline properties. A thinlayer is manufactured from a solution or a melt and is oriented in therubbery liquid-crystalline state, after which it is cooled to atemperature below the glass transition temperature. Such layers areoften turbid owing to local fluctuations in the refractive index causedby a poor orientational order. Moreover, heating the material above theglass transition temperature, even when executed only once, leads to apermanent loss of order. Besides, the method does not permit the pitchand the thickness of the polymer layer to be accurately adjusted.

In the U.S. Pat. No. 5,210,630 a polymeric film for display devices isdisclosed obtained from polymerization of a mixture of monomers havingat least two polymerizable groups in the presence of a chiral dopant.However, it is not possible to compensate the temperature dependence ofthe optical pathway of the low-molecular weight liquid crystals in thedisplay with the aid of that film.

One of the objects of the invention is to provide a polymeric film forliquid-crystal display devices being optically clear and having a largetemperature resistance. Another object of the invention is to provide asupertwisted nematic liquid-crystal display device having a highcontrast, the voltageless state being substantially completely dark andcolorless, and the voltage on state being highly transparent. Anotherobject of the invention is to provide a compensation film for a TN, aIPS, a ASM or a similar LC device.

A further object of the invention is to provide a film which can bemanufactured with the desired accuracy in a simple manner.

Another object of the invention is to provide a material which can besuitably be used in the film.

The general object of the invention is to provide a film whose opticalproperties are not too dissimilar to those of the LC within the LC cell.

According to the invention, these objects are achieved by a polymericfilm as described in the opening paragraph, characterized in that it isobtainable by copolymerization of a mixture comprising

(a) at least one monomer or oligomer, each of said monomers or oligomershaving at least two polymerizable functional groups selected from thegroup consisting of (meth-)acrylate ester, epoxy and vinyl ether,

(b) at least one achiral liquid crystalline monomer or oligomer, each ofsaid monomers/oligomers having mesogenic groups and one polymerizablefunctional group selected from the group consisting of (meth-)acrylateester, epoxy and vinyl ether,

(c) a photoinitiator,

(d) additives selected from inhibitors and accelerators,

(e) optionally at least one chiral component which is preferably aliquid crystalline compound or is compatible with liquid crystallinephases, and

(f) optionally at least one liquid-crystalline mono- or dithiolcompound.

The synthetic resin composition is preferably manufactured from acurable liquid-crystalline composition having a chiral dopant.

In a preferred embodiment of the polymeric film according to theinvention, the synthetic resin composition is formed by curing a mixtureof liquid-crystalline monomers or oligomers which consist of compounds(a) with two or more acrylate-ester groups and compounds (b) with oneacrylate ester group. Instead of acrylate compounds, epoxides, vinylethers and thiolene compounds can alternatively and satisfactorily beused as liquid-crystalline monomers.

Further preferred embodiments are:

a) A polymeric film wherein each of said monomers or oligomers of group(a) have at least two (meth-)acrylate-ester groups and each of saidmonomers oroligomers of group (b) have one (meth-)acrylate-ester group.

b) A polymeric film wherein the optically anisotropic material comprisesat least one mesogenic chiral additive (e).

c) A polymeric film preferably containing a thiol which is preferably amonothiol especially a liquid crystalline or liquid crystal-like thiol,whose purpose is to limit the molecular weight of the resultant polymer.These thiol compounds terminate the free radical initiatedpolymerization.

d) A polymeric film being obtainable by in-situ UV co-polymerization ofa mixture comprising at least one monomer or oligomer of group (a), atleast one monomer or oligomer of group (b), at least one UV initiator(c) and at least one chiral additive (e).

e) A polymeric film wherein the optically anisotropic material comprises

5-50% by weight of at least one monomer or oligomer of group (a)

20-95% by weight of at least one monomer or oligomer of group (b),

0,5-5% by weight of a photoinitiator (c),

1-20% by weight of at least one chiral additive (e), and

1-20% by weight of at least one liquid crystalline monothiol compound(f).

f) A polymeric film wherein each of said monomers or oligomers of group(a) have a mesogenic group.

It has been found in our extensive investigations that the degree ofpossible movement in a side group LC polymer is in the first approachdependent on the molecular weight of the polymers. ‘Small’ polymers havefaster response times than polymers with a high molecular weight [MWt].

In order to mimic a low molar mass LC with respect to its thermalfluctuations it is desirable for the side groups on an LC polymer to beas freely movable as possible. This is accomplished by using a low MWtpolymer which is achieved by using thiols to terminate polymerization.Typically one would have to use 10-15% of such a thiol to produce thedesired lower MWt materials.

In practice conventional thiols are not very soluble in the polymerprecursors and phase separate at 5% or even at lower concentrations(i.e. octylthiol C₈H₁₇SH). However, this problem is overcome by using aliquid crystalline thiol as e.g.

or of similar thiols.

Typical thiols used according to the present invention are of thefollowing structure:

with

n=1 to 6

m=0 to 10

e=0 or 1

k=0 or 1

6-membered 1-4 disubstituted ring which can also bear one or morelateral groups like R or F

R=C₁ to C₉ alkyl, alkenyl, oxyalkyl or oxyalkenyl.

Adding a chiral substance twists the molecular axis of the LC chain. Alayer of the material thus obtained exhibits a nematic order, with ahelical structure, also termed cholesteric order. The natural pitch ofthe mixture depends on the type and the quantity of the chiral dopantadded and is approximately 28 μm at 0.5 mol % and approximately 2.5 μmat 6 mol % for typical dopants like S-811 of Merck KGaA, Germany.

The curable synthetic composition is cured by photopolymerization usingexposure to actinic radiation e.g. ultraviolet light for typically 3minutes produced by a short-arc mercury lamp at 50° C. having aradiation intensity on the surface to be cured from 2 to 5 mW/cm².During curing the orientation is fixed, the overall rotation angle ofthe layer remaining constant. The rotation angle is a measure of thenumber of revolutions in the molecular spiral in the cholesteric nematicphase. For this reason, the variation in pitch during curing dependsonly on the change in layer thickness as a result of possiblepolymerization shrinkage.

In the polymerization operation substantially no change takes place inthe product dΔn d being the layer thickness and Δn being thebirefringence of the material. As the material shrinks in only onedirection during polymerization, the change in layer thickness isinversely proportional to the change in density of the material, whichlatter quantity is proportional to the degree of birefringence.

The rotation angle of the polymer film thus obtained exhibits notemperature dependence in the range from room temperature to 250° C.demonstrating the complete resistance to molecular reorientation as aresult of the network of polymer molecules formed by cross-linking. Asthe monomers partly contain two acrylate-ester groups per molecule,cross-linking is so strong that only little movement is possible in therigid parts of the liquid-crystalline molecules.

In comparison with, for example, cholesteric polymers having chiralgroups in side chains, the pitch has a small temperature dependence.

Within certain limits, the pitch can be influenced by curing thesynthetic resin composition between two substrates. Having a cladding ofan orienting material such a rubbed polyimide the polyimide surface isrubbed uniaxially, for example with a velvet cloth. The pitch depends onthe distance between the two substrates, according to this example, 6μm, and the angle between the two directions of rubbing the polyimidesurfaces. The number of revolutions of the molecular spiral adjustsitself such that the pitch obtained does not differ much from thenatural pitch.

However, very low MWt LC polymers melt to fluid polymers but by crosslinking the short low MWt LCP's by a small amount of a difunctionalmaterial the overall MWt can be increased but one still retains a fastor easy movement of the side groups.

We further have observed that the molecular weight of the LC polymerforming the film is not the only and may be even not the most importantparameter of the polymer influencing its wavelength dispersion of theoptical anisotropy. As this dispersion and its temperature dependenceshould be as close to that of the low molecular mass LC used in theactive (i.e. the driven) LC cell of the display which may be e.g. a TN ,a STN or another mode as e.g. in plane switching (IPS), or other LCDs,the LC polymer should be of similar nature and orientational order tothese low molecular LCs.

The probably most decisive parameter of the polymer LCs in this respectseems to be the “average linear chain length” which can be derived e.g.from the amount of the thiol used in the formulation of the precursors,considering especially its functionality and its concentration. Therelevance of this average linear chain lengths may be visualized asfollows: The low molecular weight LCs are rather freely movable, e.g.can rotate and translate more easily, compared to typical LC polymers,leading to a different dispersion and a different temperature dependenceof the dispersion. This is due to the fixation of the LC units in the LCpolymer, which may be more or less rigid from case to case but is alwaysmore rigid than that in a monomeric LC. Considering a side group LCpolymer, the LC side groups in the middle of the chain are much morerestained in their movements by the polymer backbone and the other sidegroups at each side and from similar groups in other polymer chains thanthose LC side groups close to the end of the chain. These side groups atthe end of a chain do not have such severe restrictions and consequentlybehave more like low molecular mass LC molecules. Thus the closer a sidegroup is located to the end of a chain, the more it behaves as desiredhere, i.e. like low molecular mass LC.

As the birefringent side groups of the new compensator layer shouldbehave similar to the LC cell e.g. an STN cell or maybe also an axiallysymmetric mode (ASM) cell, short polymer chains of e.g. 5 to 50 unitsand preferably of 10 to 30 and most preferred of 10 to 20 units lengthare required. These then in fact lead to a higher concentration of endgroups.

Polymer chains in the state of the art which are produced by in-situphotopolymerization have typical lengths of about 200 to 300 units.However, LC polymers with the short chain lengths desired for our typeof application very often suffer from softening which is unfavorable forpractically usable films. This suffering is diminished or even avoidedby cross-linking the short chains of the polymer together. This doesincrease the MWt of the polymer but it does not change the effectivelength of the chains and consequently does not change the end groupdensity, at least not significantly.

Thus monoreactivre thiols are beneficiously used to achieve the desiredchain lengths and/or at least direactive thiols to crosslink the shortchain polymers. Cross-linking however is preferably achieved e.g. bydireactive acrylates, epoxydes or enes.

Hence, such a polymer is an excellent compensation foil for e.g. STN andTN, in plane switching, axially symmetric mode and other AM LCDs.

Most preferred are STN and AM LCDs. The preferred mode of the AM LCDs isthe TN mode. In another embodiment the LC mode is the “in planeswitching” or IPS mode as described e.g. in EP 0 509 026 and EP 0 588568. Also the “axially symmetric aligned microdomain” or ASM mode asdescribed in EP 0 568 355, EP 0 626 607 and JP 6-301 015 A (Kokai, laidopen No.) is preferred.

Both these and other modes can be used in directly driven displays too,just as TN and STN mode.

An ordered synthetic resin composition can be obtained, for example, byorienting a liquid-crystalline monomer mixture and freezing saidorientation by exposure to UV light in the presence of a light-sensitiveinitiator, as in principle disclosed by EP 0 445 629.

A chiral component (e), for example, a compound with an asymmetricallysubstituted carbon atom can be added to the monomer. This dopant bringsabout a rotation of the monomer molecules in a direction perpendicularto the longitudinal axis of the molecules. By arranging the monomerbetween two polyimide-coated and rubbed surfaces or other orientingsurfaces such as obliquely deposited SiO, the degree of orientation orrotation can be adjusted as a function of the natural pitch (the pitchwithout the presence of such surfaces), the distance between the rubbedsurfaces and the direction of rubbing of the surfaces. Subsequently, thecrystalline rotation in the still liquid monomer composition is fixed bypolymerization of the reactive end groups under the influence of UVlight or irradiation using electrons. The desired order is rapidlyobtained and is substantially perfect, so that a clear film or thinlayer is attained. As a result of the use of a mixture of monomershaving at least two functional groups (a) and of monomers having onlyone functional group (b) an ordered polymer network is maintained up tovery high temperatures.

Compounds suitable as chiral dopants for the polymer precursor are forexample:

Other chiral dopants are known to the experts. In a preferred embodimentof the present invention an enantiomeric pair of chiral molecules isused as dopants, one enantiomer for low molecular weight LC in the LCcell and its mirror molecule, the enantiomer with the oppositeconfiguration, for the polymer precursor. Such pairs are known to theexpert. A typical example is S-811 and R-811 of Merck KGaA, Germany,where S-811 has negative helical twisting power inducing left-handedtwist. Also combinations of chiral dopants are used with beneficiouseffects, e.g. reducing the temperature dependence of the pitch.

Since the mixtures of monomers or oligomers for obtaining the polymericfilm according to the present invention exhibit, as a rule, broadnematic or cholesteric mesophases with melting points at comparable lowtemperatures, the inventive film may be produced by irradiation atcomparable low temperatures (below 100° C. preferably between 30 and 80°C.). If desirable, the curable synthetic resin composition may comprisea mixture of various oligomeric compounds. Besides, the synthetic resincomposition may comprise one or more other suitable components such as,for example, catalysts, (light-sensitive) initiators, stabilizers,co-reacting monomers and surface-active compounds. It is alternativelypossible to add, for example, a quantity of up to 50% by weight of anonpolymerizable liquid-crystalline material to adapt the opticalproperties of the material.

Suitable compounds (a) are known from U.S. Pat. No. 4,758,447. In theapplication described although, no helicoidal order is pursued. A methodof manufacturing suitable compounds is described in European Patent 0261 712. Suitable compounds (a) and (b) are disclosed for example in WO93/22 397; further suitable compounds (b) are disclosed for example inEP 0 590 376.

As a rapid curing of the composition is desired, the curing operation isinitiated, preferably, by means of actinic radiation. The expressionactinic radiation is to be understood to mean herein radiation usinglight, in particular UV light, X-rays, gamma rays or radiation usinghigh-energy particles such as electrons or ions. The geometry of thedisplay device, in particular a TN, STN, IPS or ASM device, isconventional.

Films according to the present invention are easily modified withrespect to adaption of the optical retardation dΔn to that of the LCcell to be compensated, e.g. by modification of the layer thickness orthe composition of the polymer precursor, which are both variable over awide range. The adaption can be achieved either to the on states or tothe off states of the active cells or to an intermediate partiallyswitched state, as desired the desired twist angle of the molecules ofthe compensation film can be achieved by modification of the chiraldopant used and its concentration. Especially preferred are singlechiral compounds, which are cholesteric liquid crystals or arecompatible with the liquid crystalline precursor of the polymer film ofthe compensator. Especially preferred are single chiral compounds orcombinations of chiral compounds of preferably two and less preferablymore than two chiral compounds which have a small temperature dependenceof the cholesteric pitch in the polymer precursor, as they do not leadto a very stringent requirement for the temperature control during thecuring of the polymer precursors. They further allow to vary the curingand processing temperatures easily and over wide temperature rangeswithout influencing the resultant pitch and consequently the twist ofthe compensator.

The films do not have to be contained between two substrates for curing.In a preferred embodiment free standing thin films supported by onesubstrate with an orientation layer are cured.

EXAMPLES Example 1

A starting mixture for a curable composition is manufactured from 84parts by weight of a mono acrylate compound A

15 parts by weight of a diacrylate compound B

and 1 part by weight of a light-sensitive initiator, in the presentexample 2,2-dimethoxy-2-phenyl-acetophenone, commercially available fromCiba-Geigy under the trade name Irgacure® 651 (IRG 651). A method ofmanufacturing the diacrylate compounds is described in EP-0 261 712 andfor the monoacrylate in EP 0 590 376. The starting mixture additionallycomprises 100 ppm of a stabilizer, for example hydroquinone monomethylether.

The starting mixture is a composition having a melting point of 38.2°C., above which exists a nematic phase which changes into an isotropicphase at a temperature of 123.3° C. The mixture is used, coated andcured at a temperature between these two temperatures, the highestviscosity and highest molecular order being obtained at the lowesttemperatures.

The mixture is cured at 80 ° C. with 6 mW/cm² at 365 nm (+/−7 nm FWHM)after spincoating and evaporation of the solvent. The resulting film hasno chiral twist or in other words an infinite cholesteric pitch as nochiral additive had been used.

Example 2

According to the present example, different quantities of a chiraldopant can be added to the starting mixture, for example left-handed4-(4-hexy-loxy-benzoyloxy)-benzoic acid-2-octylester commerciallyavailable from Merck under the number S-811. The chiral dopant itselfmay not have to exhibit liquid-crystalline properties, and may be both acopolymerizing monomer and a nonpolymerizable compound. To obtain atwisted nematic order in the polymer it is sufficient for the compoundto be chiral. The chiral dopant may be left-handed or right-handed.

For Example 2 14% of the diacrylate B were used together with 84.2% ofthe monoacrylate A, 1% of the photoinitiator IRG 651 and 0.8% of thechiral dopant S-811, all concentrations are given in mass%. It was curedafter solvent (toluene) evaporation, under dry nitrogen at a temperatureof 75° C. for 10 minutes with 5 mW at 370 nm (+/−10 nm). The results arelisted in the following Table 2.

The films prepared were applied to typical STN cells all with theappropriate twist angle of either 180°, 240° or 270° and 5° surfacepretilt angle (20 to 30 pretilt for 180° twist) filled with the liquidcrystal mixture ZLI-2293, a sales product of Merck KGaA of Germany,doped with the chiral dopant R-811, i.e. the enantiomer of S-811 to ad/p value of 0.5 or 0.55 for the 270° twisted cells. The matchrespectively mismatch of the dispersions were observed both bydetermining the dispersion of the film and the LC cell individually aswell as by that of the combined display element. The Example 1 as wellas Examples 2 and 3 can be looked upon as comparative examples. Theprecursor components used are summarized in Table 1, whereas the resultsare listed in Table 2.

Examples 3 to 8

These examples contain some thiol to limit the MWt and the averagelinear chain lengths of the LC polymer of the compensation film.

The films were made like for Example 1 by solvent coating i.e. spincoating solutions of the components in toluene onto a rubbed polyimidesurface, subsequent evaporation of the solvent at an elevatedtemperature followed by UV exposure under a nitrogen atmosphere at 70 to80 ° C., of λ=360 to 380 nm, 5 to 20 mW/cm².

TABLE 1 Polymer Precursors Compound n Structure Diacrylate A 3

Diacrylate B 6

Monoacrylate B 6

Thiol A 6

Photoinitiator IRG 651 —

TABLE 2 Results for Examples Examples EX 2 EX 3 EX 4 EX 5 EX 6 EX 7 EX 8(a₁) diacrylate A (%) — 13.9 5 5 3 5 — (a₂) diacrylate B (%) 14 — — — 2— 5 (b₁) monoacrylate A 84.1 — — — — — — (%) (b₂) monoacrylate B — 8487.9 82.9 82.9 87.9 82.9 (%) (c) thiol A (%) — — 5 10 10 5 10 (d) IRG651 (%) 1 1 1 1 1 1 1 (e) S-811 (%) 0.8 1.1 1.1 1.1 1.1 1.25 1.1 Avlinear chain length ˜200 ˜200 20 10 10 20 15 Film thickness 6 μm 6 μm 6μm 6 μm 6 μm 6 μm 6 μm Helical twist 180° 240° 240° 240° 240° 270° 240°Dispersion Match LC poor poor good good good good good Temp. dep. poorvery mod- good good good mod- poor erate erate

These examples can easily be varied by the expert. For example by usinga smaller concentration of the chiral dopant the twist of thecompensating layer can be adjusted to 90° matching a typical TN device.The adaption of the optical and chiral properties of the compensator tovarious types of LCDs including amongst others IPS and ASM devices isobvious to the expert from their respective molecular arrangements inthe cells either in the off, the on or any partially switched state.

What is claimed is:
 1. A monothiol compound of the formula

wherein n=1 to 6 m=0 to 10 e=0 or 1 k=0 or 1

 6-membered 1,4-divalent ring which is optionally substituted by one ormore lateral R or F groups, and R=C₁ to C₉ alkyl, alkenyl, oxyalkyl oroxyalkenyl.
 2. A compound according to claim 1 of the formula

wherein n=1 to
 6. 3. A monothiol compound of claim 1, wherein k is 1 andone of A and B is 1,4-cyclohexylene and the other is 1,4-phenylene thatis optionally substituted by one of more lateral R or F groups.
 4. Amonothiol compound of claim 1, wherein k is 1 and A and B are1,4-phenylene that is optionally substituted by one or more lateral R orF groups. substituted by one or two R or F groups.
 5. A monothiolcompound of claim 4, wherein at least one phenylenene ring issubstituted by one or two R or F groups.
 6. A monothiol compound ofclaim 1, wherein n is
 2. 7. A monothiol compound of claim 1, wherein eis 1.